1. Field of the Invention
The present invention generally relates to a sheet feeder used in an image forming apparatus, such as a copier, a printer, a facsimile machine, or a multifunction machine including at least two of these functions, and an image forming apparatus including the sheet feeder.
2. Discussion of the Background Art
There are image forming apparatuses in which a long sheet unreeled from a paper roll by a feed roller is conveyed by a registration roller to an image forming unit at a predetermined timing. Such image forming apparatuses generally include a guide member disposed along a sheet conveyance path to guide the sheet so that the sheet does not go slack.
For example, JP-2005-343657-A discloses a sheet feeding mechanism that includes a guide plate movable relative to a main body of the image forming apparatus to prevent the sheet unreeled from the paper roll from becoming slack in the sheet conveyance path.
Recently, in response to demand for faster image formation speeds, it is preferred that the registration roller can start up immediately to send the sheet to the image formation unit quickly. By contrast, as the moment of inertia of the paper roll is relatively large, it is preferred that the feed roller is started up gradually considering torque margin. In other words, it is preferable that a registration motor for driving the registration roller should start up immediately, whereas a feed motor for driving the feed roller should start up gradually.
With this configuration, the rotational velocity at start-up differs between the registration roller and the feed roller if the two rollers are started simultaneously, and consequently the registration roller transports the sheet faster than the feed roller feeds the sheet out. As a result, the sheet is subjected to force in the reverse direction to the direction in which the sheet is transported, and a force that stretches the sheet backward (back tension) increases abruptly. As the back tension exerted on the paper roll increases, that is, the force pulling the sheet in the reverse direction increases, the sheet is more likely to slip on the registration roller. Slippage of the sheet on the registration roller can result in sheet conveyance failure. For example, the registration roller might fail to transport the sheet at the predetermined timing to the image forming unit.
Although capable of preventing slackage of the sheet in the sheet conveyance path to some extent, the above-described related-art sheet feeder does not address the slippage of the sheet on the registration roller.
In view of the foregoing, the inventors of the present invention recognize that there is a need for a sheet feeder capable of preventing both slippage of the sheet on the registration roller and slackage of the sheet, which known approaches fail to do.